1. Field of the Invention
The present invention relates to a magnetic head and core chip and more particularly to a magnetic head for writing and/or reading data onto and/or from a magnetic recording medium such as a metal tape in a video tape recorder (VTR), a digital audio tape recorder (DAT), a floppy disc drive (FDD) or the like.
2. Description of the Prior Art
Currently, units of the types described above tend to be compact with a high degree of picture quality and high fidelity in sound reproduction. This is consistent with the fact that magnetic recording medium have been reduced in size, shorter in recording wavelength and increased in packing density. Therefore, a so-called metal tape, which uses ferromagnetic metal powder as magnetic particles bonded thereon, and characterized by high coercivity and high residual magnetic flux density, has been used.
The materials of the magnetic head for writing and/or reading data onto and/or from the magnetic recording medium described above, need to have a high magnetic saturation density and to maintain high permeability in a high frequency range. As an example of a magnetic head made of the materials described above, well known is a so-called metal-in-gap type composite magnetic head (to be referred to hereinafter as a "MIG head") in which the core is mainly made of ferrite and in which thin films of a soft magnetic material such as Sendust having a high magnetic saturation density are formed in the vicinity of a gap.
In other words, for writing data onto a magnetic recording medium such as a metal tape, the magnetic field formed by the magnetic head needs to have a strength two or three times as high as the coercivity of the metal tape so that the materials for the magnetic head must have a high magnetic saturation density. For instance, the magnetic saturation density of a ferrite is of the order of 4500.about.5000 gauss, and when the coercivity of the magnetic recording medium is in excess of 1000 oersteds, it becomes impossible to effectively write data onto the magnetic recording medium. On the other hand, in the case of magnetic heads made of metal magnetic materials such as crystal alloys; Fe-Al-Si alloy (Sendust), Ni-Fe alloys (Parmaloy) and the like, or amorphous or noncrystal materials; Co-Nb-Zr, Co-Ta-Zr, Co-To-Hf and the like, their magnetic saturation density is generally higher than that of ferrite and the noise produced when the head is brought into sliding contact with the surface of the recording medium is less. However, the materials described above have a problem that, when the thickness of the film is in excess of 10 .mu.m, the effective permeability at a high frequency (for instance, 5 MHz) becomes lower than that of ferrite because of eddy current loss so that the reproduction efficiency becomes low. In addition, the durability of the materials becomes less than that of ferrite. In view of the above, in MIG head, for mutually compensating for the defects of each of the various materials described above and the ferrite, in the case of making up a magnetic core, the ferrite and the metal magnetic material are combined. More specifically, the main body of the magnetic core is made of ferrite and magnetic thin films made of the metal magnetic material are grown in the vicinity of the gap by a film deposition in vacuum, whereby the magnetic core can be obtained.
FIG. 1 is a plan view illustrating the slide surface of a conventional magnetic head with which a surface of the magnetic recording medium contacts. In FIG. 1, reference numerals 1 and 1' represent ferrite portions; 4 and 4', soft magnetic thin films; 5 and 5', glass portions for joining the ferrite portions 1 and 1'; and g, a magnetic gap. A predetermined angle is defined between the magnetic gap g and a plane defined by the soft magnetic thin films 4, 4' over the slide surface, so that in case of poor magnetic layers which act as magnetic gaps being formed at the interfaces between the ferrite portions 1 and 1' and the soft magnetic thin films 4, 4', the undulation in the electromagnetic conversion characteristics (contour effect) due to the mutual interference of the signals derived from these gaps and the signal derived from the true gap g can be prevented. However, the production of such a magnetic head is complicated. Moreover, since the track width defined by the length of the gap g is determined by the thickness of the soft magnetic thin films 4, 4', in order to increase the width of the track, the thickness of the films 4, 4' must be increased (to 20-30 .mu.m). As a result, the production cost of such a magnetic head rises.
FIG. 2 is a plan view illustrating the slide surface (the magnetic core) of another conventional MIG head. Soft magnetic thin films 8 and 8' are formed over the whole surfaces in mutually opposing relationship with each other on ferrite portions 1 and 1' comprising core body halves of a core body. In this case, the surfaces of the ferrite portions 1 and 1' over which the soft magnetic thin films 8 and 8' are formed in the vicinity of the magnetic gap g are substantially in parallel with the gap g. As a result, in case of increasing the width of the track, the thickness of the films 8, 8' remain thin (5-10 .mu.m), and production of the films can be simplified. It has been apparent that by the arrangement of the magnetic core shown in FIG. 2 the contour effect can be reduced to a minimum by, for instance, suppressing the mutual interference between the ferrite portions 1, 1' on the one hand and the soft magnetic thin films 8, 8' on the other hand.
A magnetic head with the arrangement shown in FIG. 2 has many advantages, but there exists a problem that part of the magnetic core tends to be damaged during the production process.
More specifically, in the production of magnetic heads of the type described above with reference to FIG. 2, a block which has a plurality of arrangements similar to that shown in FIG. 2 extended repeatedly in the direction in parallel with the gap g is sliced to obtain a plurality of magnetic core chips. In this case, the soft magnetic thin films, the ferrite portions, and the glass portions are simultaneously sliced. As the strain caused by thermal stresses due to the difference in the coefficients of expansion between the films and the ferrite portions when the ferrite portions 1 and 1' are bonded together by the melted glass 5, 5' are released, cracks occur as indicated by 6 in FIG. 2 between the thin films and ferrite portions.
Next, a second problem will be described. In the case of a magnetic head of the type shown in FIG. 2, the glass portions 5, 5' and the soft magnetic thin films 8, 8' are in contact with each other through a magnetic gap material such as SiO.sub.2. An example of this type of magnetic head is shown in FIG. 1 in "AUGER SPECTROSCOPY ANALYSIS OF METAL/FERRITE INTERFACE LAYER IN METAL-IN-GAP MAGNETIC HEAD; IEEE TRANSACTIONS ON MAGNETICS, Vol 24, No. 6, November 1988". In this arrangement, the bonding strength between the glass portions and the soft thin magnetic films is relatively weak causing a problem to arise in that the strength of the magnetic core chip becomes weak. For instance, in the production of magnetic heads, the sliced chips are subjected to a washing step, and during this step, due to the influence of vibration, about 5% of the core chips are cracked at the joint between the halves of the core. In addition the core chip is subjected to a step in which it is bonded to the head base, and in this step about a few percent of the core chips are cracked.
The strength of the magnetic core chips shown in FIGS. 1 and 2 was confirmed by clamping only one half of the core chip and measuring the force applied to the other half so as to break the chip. The experimental results are as follows. In the case of the head shown in FIG. 1, the average breaking strength of 20 samples was 270 g.multidot.f, but in the case of the head shown in FIG. 2, the average breaking strength was 130 g.multidot.f or less than one half of the average breaking strength of the head shown in FIG. 1.
From these experimental results, it becomes clear that the greater the area of the interface between the soft magnetic thin films and the glass portions, the weaker the joint strength between the magnetic core halves. In addition, it is confirmed that all the breakages occur along the interfaces between the soft magnetic thin films and the glass portions.
Next, a third problem encountered with a magnetic head of the type shown in FIG. 2 will be described. The core of such a compound type magnetic head comprises ferrite portions 1 and 1' over the surfaces of which the soft magnetic thin films 8 and 8' of Sendust, amorphous or the like are formed so that due to the difference in the coefficients of thermal expansion between them and in the degree of heat resistance of each material, the temperature of the glass 5, 5' being melted for bonding is limited to less than 600.degree. C. For instance, the coefficient of linear expansion a of Mn-Zn single crystal ferrite has an average value of 110.times.10.sup.-7 /.degree.C. in the temperature range 30.degree.-500.degree. C. while Sendust has an average coefficient of linear expansion of 160.times.10.sup.-7 /.degree.C. in the range from 30.degree.-500.degree. C. When a thermal hysteresis in excess of 600.degree. C. is applied, due to the thermal stresses caused by the difference in the coefficients of linear expansion, they tend to crack the ferrite portions and the glass. In the case of a soft magnetic thin films composed of an amorphous alloy, the alloy crystalizes at 500.degree..about.550.degree. C. so that the bonding by glass cannot be carried out in excess of the above-mentioned temperature. More specifically, when the temperature of the glass melting is in excess of the above-mentioned temperature range in the bonding process, the soft magnetic property of amorphous alloys is lost so that they cannot be used as materials for magnetic heads.
On the other hand, when the temperature of the glass melting is low, the properties of glass such as water-resistance, weatherability, strength and so on is adversely affected. The reason is as follows. For instance, in the case of the glass of PbO series, in order to lower the welding temperature, the content of the PbO must be increased while the content of SiO.sub.2 must be decreased. In this case, SiO.sub.2 is one of the compounds subjected to vitrification and if SiO.sub.2 is decreased in quantity, the water-resistance, weatherability, anti-environmental-pollution ability and so on are also adversely affected. It follows therefore that, for improving the anti-pollution ability, the first requirement is to increase the temperature in the bonding process (bonding temperatures). In general, in the case of PbO and V.sub.2 O.sub.5 series glass, the higher the softening temperature, the more the anti-pollution ability is improved. However, in the case of the above-described magnetic head, the temperature of glass at melting cannot be increased, and it becomes impossible to use glass having a high softening temperature so that the anti-pollution ability of the glass used for joining the core halves cannot be satisfactorily improved.
In addition, in the case of PbO series glass, B.sub.2 O.sub.3 is included for the sake of vitrification while Bi.sub.2 O.sub.3 is included in order to lower the softening temperature. As compared with PbO, the degradation of the anti-pollution ability is low. It follows that in the case of PbO series glass, PbO-Sio.sub.2 -Bi.sub.2 O.sub.3 -B.sub.2 O.sub.3 series glass has relatively an excellent degree of water-proofness.
Sometimes ZnO and Al.sub.2 O.sub.3 are added. In this case, ZnO is effective to improve the water resistance but its wearability is degraded. Al.sub.2 O.sub.3 improves water resistance, but the softening temperature rises. Therefore it is preferable to combine them at a suitable ratio.
Furthermore, when the halves of a core chip are bonded to each other by glass, firstly, glasses are disposed into a winding groove and a groove defined at the rear portion of each of the cores respectively, and the melted glass flows therein. The distance for which the glass disposed into the groove at the rear portion (to be referred to hereinafter as the back-gap-side glass) flows is considerably longer than the distance for which the glass disposed into the winding groove (to be referred to hereinafter as front-gap-side glass) flows. On the other hand, the above-described glass must have a softening temperature which is as high as possible to optimize the anti-pollution ability of the glass, and must have a low bonding temperature to prevent cracks during the production process. Therefore the bonding temperature must be set at a temperature as low as possible for the front-gap-side glass to reach to the predetermined point by flowing. However, when the glass bonding is carried out at such temperature, sometimes the back-gap-side glass cannot reach a predetermined position. As a result, there arises the problem that the strength of the core chip is decreased and the core chips are cracked during the production process.